level-updater [](http://travis-ci.org/hughsk/level-updater)

Pseudo-atomic update methods for levelup. Started out as level-inc, but this is more useful.

Why? If you're making a lot of changes dependent on the current value of a key, you'll find the gap between a get and a put becomes important:

for (var i = 0; i < 100; i += 1) {
  db.get('a-key', function(err, val) {
    val = parseInt(val || 0, 10)
    val += 1

    db.put('a-key', val)
  })
}

The above example will probably result in a-key being set to 1 and staying there. level-updater keeps track of overlapping calls like this and handles them cleanly for you.

Installation

npm install level-updater

Usage

require('level-updater')(db, updater)(key, [param], [callback])

When initializing:

• db is the levelup instance to plug into.
• updater is the method used to update each time a value is hit.

And for each call:

• key is the key to update.
• param is an optional parameter for passing to updater.
• callback is called after the new value has been stored in the database.

Take this example for incrementing numbers:

var db = require('levelup')(__dirname + '/db')
var update = require('level-updater')
var counter = 0

db.inc = update(db, function(value, param, key) {
  return (value | 0) + param
})

for (var i = 0; i < 200; i += 1) {
  db.inc('some-key', 10, function() {
    counter += 1
    if (counter !== 200) return
    db.get('some-key', function(err, val) {
      // val === "2000"
    })
  })
}

And this for merging JSON objects:

var db = require('levelup')(__dirname + '/db')
var update = require('level-updater', {
  valueEncoding: 'json'
})

db.merge = update(db, function(original, merger) {
  return Object.keys(param).reduce(function(current, key) {
    current[key] = param[key]
    return current
  }, value || {})
})

db.merge('doc', { hello: 'world' })
db.merge('doc', { lorem: 'ipsum' }, function(err) {
  db.get('doc', function(err, val) {
    // val === { hello: 'world', lorem: 'ipsum' }
  })
})